[ntp:questions] NTPD can take 10 hours to achieve stability

David Lord snews at lordynet.org
Wed Apr 20 21:56:21 UTC 2011


unruh wrote:
> On 2011-04-20, David Lord <snews at lordynet.org> wrote:
>> unruh wrote:
>>> On 2011-04-20, David Lord <snews at lordynet.org> wrote:
>>>> unruh wrote:
>>>>> On 2011-04-19, David Lord <snews at lordynet.org> wrote:
>>>>>> unruh wrote:
>>>>>>> On 2011-04-19, David Lord <snews at lordynet.org> wrote:
>>>> .....
>>>>
>>>>>>>> When I first compared chrony with ntpd there was no contest
>>>>>>>> but more recent experiments with chrony had periods of
>>>>>>>> severe instability much worse than ntpd.
>>>>>>> More recent means what? What version of chrony? (there was a bug found
>>>>>>> in the past two weeks that was introduces a few months ago which did
>>>>>>> result in instability) 
>>>>>> More recent than  November 2009.
>>>>>> Probably between Dec 2009 and Jan 2010 and p4x2400c.
>>>>>>
>>>>>> chrony 1.23
>>>>> OK well before that change. 
>>>>>
>>>>>>>> P4X2666 with chrony
>>>>>>>> <http://www.lordynet.org.uk/mrtg/stats/>
>>>>>>> Not at all sure what I am supposed to see. I have no idea what the graph
>>>>>>> axes represent?  What is 1.1k 
>>>>>> X-axis is in hours
>>>>>> Y-axis is offset in us so 1.1k = 1100us
>>>>> WOW. What kind of network are you attached to? Even on an ADSL link
>>>>> through the phone company, I
>>>>> was getting in the tens of usec (not ms) as the offsets of chrony.
>>>>> (checked by a gps receiver attached to the local computer).
>>>> I'm on ADSL-1 with 2 Mbit/s down and 288 kbit/s up. Latency
>>>> is what BT delivers and has been as low as 12ms up to 50ms or
>>>> more but mostly it's about 18ms to nearest sites (when exchange
>>>> gets near capacity the latency jumps up by 15ms or more same as
>>>> when interleave is enabled on ADSSL-2).
>>>>
>>>> Although the peaks of the graph are at about 1ms the stats
>>>> have "System time" mostly being in the 10s of usec.
>>> Not sure what you mean by that. What I mean is , what are the measured
>>> offsets? (Of course if you have a GPS PPS that you could use as a
>>> reference-- not a source of time-- that would make much clearer what is
>>> going on.)
>> My logs have eg:
>> Reference ID    : me6000g
>> Stratum         : 2
>> Ref time (UTC)  : Sat Jan  9 07:12:02 2010
>> System time     : 0.000001 seconds slow of NTP time
> 
> OK, that is the kind of thing chrony should do. Ie, to discipline the
> system clock to usec levels
> 
>> Frequency       : 0.125 ppm fast
> 
> That is a really small drift rate. Just lucky I guess. 
> 
>> Residual freq   : -0.010 ppm
>> Skew            : 0.223 ppm
> So the frequency is really pretty stable. 
> 
>> Root delay      : 0.001297 seconds
>> Root dispersion : 0.007492 seconds
> 
> That is a huge root delay and dispersion. Where are your servers?
> 

Possibly the pc using a MSF 60kHz radioclock is preferred vs
the internet servers. Reception during December through March
can be bad. GPS is also affected by same weather conditions.
Internet connection was also bad during winter 2009/2010 with
electric and gas supplies both failing.

> 
>>
>> I've not yet checked all of the systems that were tried
>> with chrony but these servers are all in use so it's not
>> easy to just install chrony and expect it to work.
> 
> Sure it is. Just install it, configure it and start it ( switching off
> ntpd first). Of course if your server is supposed to keep really good
> time all the time, then this might cause some initial problems as you
> switch over. 

I run a couple of internet servers and added them to ntp pool
which is why I'm not keen to take risks.


David




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